Add execute plugin and accompanying shell scripts.

These accompanying plugins (non-traditional shell plugins) fill out the
missing functionality from rs-sysmon and should complete feature parity
of the two softwares once again.

At this point testing is missing.  The following need to occur before
contemplating the first official release of this software:

1. Add unit tests
2. Fix all found issues
3. Generate default configuration file
4. Verify packages run pmort as a daemon
5. Release 1.0 after user acceptance.